Biomedical tests are often based on the detection of an interaction between a molecule, which is present in known amount and position (the molecular probe), and an unknown molecule to be detected or unknown molecules to be detected (the molecular target molecules). Typically, probes are laid out in the form of a substance library on supports, the so-called microarrays or chips, so that a sample can be analyzed simultaneously at various probes in a parallel manner (see, for example, J. Lockhart, E. A. Winzeler, Genomics, gene expression and DNA arrays; Nature 2000, 405, 827-836). The probes are herein usually immobilized on a suitable matrix, as is for example described in WO 00/12575 (see, for example, U.S. Pat. No. 5,412,087, WO 98/36827), or synthetically produced (see, for example, U.S. Pat. No. 5,143,854) in a predetermined manner for the preparation of the microarrays.
In general, a target molecule labeled with a fluorescence group in the form of a DNA or RNA molecule to a nucleic acid probe of the microarray, that both target molecule and probe molecule are present in the form of a single-stranded nucleic acid. Efficient and specific hybridization can only occur between such molecules. Single-stranded nucleic acid target molecules and nucleic acid probe molecules can normally be obtained by means of heat denaturation and optimal selection of parameters like temperature, ionic strength, and concentration of helix-destabilizing molecules. Probes having virtually perfectly complementary, e.g. corresponding to each other, sequences remain paired with the target sequence (A. A. Leitch, T. Schwarzacher, D. Jackson, I. J. Leitch, 1994, In vitro Hybridisierung, Spektrum Akademischer Verlag, Heidelberg/Berlin/Oxford).
A typical example for the use of microarrays in biological test methods is the detection of microorganisms in samples in biomedical diagnostics. Herein, it is taken advantage of the fact that the genes for ribosomal RNA (rRNA) are dispersed ubiquitously and have sequence portions, which are characteristic for the respective species. These species-characteristic sequences are applied onto a microarray in the form of single-stranded DNA oligonucleotides. The target DNA molecules to be examined are first isolated from the sample to be examined and are equipped with markers, for example fluorescent markers. Subsequently, the labeled target DNA molecules are incubated in a solution with the probes fixed on the microarray; nonspecifically occurring interactions are removed by means of corresponding washing steps and specific interactions are detected by means of fluorescence-optical evaluation. In this manner, it is possible to detect, for example, several microorganisms simultaneously in one sample by means of one single test. In this test method, the number of detectable microorganisms theoretically only depends on the number of the specific probes, which have been applied onto the microarray.
The use of microarrays or probe arrays is not limited to the detection of target-probe interactions between nucleic acid molecules. Targets can, for example, also be proteins, which are detected by means of specific antibodies functioning as probes. In the same manner, interactions between a protein and low-molecular chemical compounds can be identified if the protein is immobilized on the array in form of a target and the chemical compounds, which can, for example, be a substance library, are immobilized on the array in the form of probes.
Targets can also be analyzed with the aid of conventional immunoassays (for example ELISAs). For instance, antibodies can be immobilized on the base of a well of a micro well plate. Subsequently, a blood sample to be analyzed is fed into said well. If the corresponding antigen is present in the blood sample, it will bind to the immobilized antibody and can then be detected, for example, via a second antibody bearing a fluorescence label.
In the case where cells are to be detected as targets or, for example, the presence of specific antigens on the surface of cells is to be analyzed, cytometric methods are often employed. Said methods are conventionally based on the fact that corresponding, for example fluorescence-labeled, antibodies are added to the sample to be analyzed. Said antibodies then bind to the surfaces of the cells or to the antigens presented there. The sample treated in this manner is subsequently guided through a corresponding capillary in a suitable device and in a suitable solution. Next to the capillary, a detector is arranged, which detects how often within a specific time interval a signal is triggered by a labeled cell flowing by. The number of cells wanted per volume unit can then, more or less exactly, be determined from the fluctuation rate and the signals counted.